One of the most important issues to consider when specifying cable cleats is the risk of material corrosion – not just as a result of the installation environment, but also from other metals which the cable cleat may come in to contact with.

Galvanic Corrosion

Galvanic corrosion occurs when dissimilar metals are placed in contact with each other in the presence of an electrolyte. There are two factors that affect the rate of galvanic corrosion; the first is the distance between the two metals in the galvanic series.

The further apart the two metals are in the series, the greater the risk of galvanic corrosion – with the metal higher up the list (more anodic) being the one whose rate of corrosion is accelerated.

The second factor to consider is the relative surface areas of the different metals.

If the more anodic (higher up the list) metal has a smaller surface area than the metal it is in contact with, the difference in surface area causes the rate of corrosion of the anodic metal to increase.

Conversely, if the more anodic metal has a much larger surface area than the cathodic metal, it may be sufficient for the effects of galvanic corrosion to be discounted.

In terms of cable cleat selection, the surface area of the cable cleat is generally significantly smaller than the structure it is mounted on.

Therefore, if it is made from a metal that is more anodic than its support structure it will be susceptible to galvanic corrosion.

Conversely, if the cable cleat is more cathodic than its support structure, there is little risk of galvanic corrosion.

Using this criteria, if galvanised ladder is the support structure, and there are no other significant factors, it is safe to use stainless steel or aluminium cable cleats. However, if the support structure is stainless steel, separation should be provided if aluminium or galvanised cable cleats are used.

Galvanic corrosion is not easily predictable and can be influenced by the type of electrolytes present such as salt water or fresh water containing impurities.

In general terms when guarding against galvanic corrosion, the safest course of action is to separate dissimilar metals with polymer separation washers.

This separation should be carried out between the cable cleat and its mounting surface and the cable cleat’s mounting fixing.

All Ellis products constructed from dissimilar metal are designed in a way that completely avoids bimetallic contact. As a result of this you can be confident that cable cleats will have a service life measured in decades.

Stainless Steel

In general, cable cleats are manufactured from austenitic stainless steel due to its non-magnetic and corrosion resistant properties – the former ensuring the cable cleat won’t induce eddy currents or localised heating of the cable.

Austenitic stainless steel does become a little magnetic as a result of work hardening when processed. This magnetism can barely be detected with a magnet and so is not significant from an eddy current perspective.

There are many different types of stainless steel, but there are two principal variants when it comes to cable cleats.

304 austenitic stainless steel, often referred to as A2, is one of the most commonly used stainless steels. It has excellent corrosion resistant properties in most circumstances, although is susceptible in atmospheres where chlorides are present, making it unsuitable for use in coastal or marine environments.

316 austenitic stainless steel, often referred to as A4, contains Molybdenum, which provides resistance against chlorides. 316 is often referred to as marine grade stainless steel due to its suitability for use in coastal and offshore applications.

If unsure a simple chemical test can determine whether Molybdenum is present and so differentiate between 304 and 316.

304 and 316 stainless steel are available in low carbon variants, namely 304L and 316L. These variants are immune to sensitisation (grain boundary carbide precipitation).

Any cable cleat which is manufactured from stainless steel and includes welding in the manufacturing process should be made in a low carbon (L) variant.

ALWAYS REMEMBER: All Ellis stainless steel cable cleats are produced from 316L austenitic stainless steel.

Coatings

The corrosion resistance properties of stainless steel are a result of Chromium, which reacts with Oxygen and forms a self-healing impervious layer of Chromium Oxide on the surface of the steel.

In most circumstances the Chromium Oxide layer is extremely durable and helps in resisting galvanic corrosion. However, in certain installation locations, such as railway tunnels, the Oxide layer can be continuously penetrated. This occurs due to trains frequently applying their brakes, which releases mild steel dust into the atmosphere that then settles on the stainless steel. If moisture is present, then corrosion occurs at an exaggerated rate.

In such circumstances, if regular washing is not feasible, use of aluminium as an alternative to stainless steel products and/or coating processes are strongly recommended.

Ellis offers special coatings to suit specific environments – e.g. our London Underground Approved electrostatic plastic coatings.

Fixings

Closure fixings on cable cleats are fundamental to the loop strength of the cable cleat and its short-circuit withstand capability.

All Ellis 316L stainless steel cable cleats use 316 fixings, which are manufactured to a precise and specific tensile strength. Fixings are sourced directly from approved manufacturers and any fixing on any cable cleat is directly traceable back to the batch quality records at that manufacturer.

Galvanised Steel

Contracts often require a guarantee regarding the life expectancy of a cable cleat.

If the installation is designed correctly and all other corrosion issues have been considered this is a relatively simple exercise for stainless steel products.

With galvanized steel, life expectancy is determined by the thickness of the zinc coating. The resistance of galvanizing to atmospheric corrosion depends on a protective film that forms on the surface of the zinc.

When the newly coated steel is withdrawn from the galvanizing bath, the zinc has a clean, bright, shiny surface. With time a corrosion process occurs which produces a dull grey patina as the surface reacts with oxygen, water and carbon dioxide in the atmosphere. This leads to the formation of a tough, stable, protective layer, which is tightly adherent to the zinc.

As the corrosion process is continuous, the thickness of the zinc layer reduces over time and it is the speed of this reduction that is used to accurately predict the life span of the cable cleat.

Corrosion rates for the UK

Permission to use the information relating to galvanising was granted by the Galvanizers Association for galvanised steel.

If a galvanised steel cable cleat is specified for use in a zone 3 area then the corrosion rate is 1.5 microns (µm) per year.

If the contract for this specification states a required life expectancy of 40 years, then the initial galvanising thickness will need to be a minimum of 60 µm in order to meet the required longevity.

ALWAYS REMEMBER: The corrosion rate for zinc is generally linear for a given environment.

Where the system peak fault current and the cable diameter are known, the following formula, taken from the International standard (IEC 61914), can be used to calculate the forces between two conductors in the event of a three phase fault:

Ft = 0.17 x ip 2 / S

Where:

Ft = force in Newton/metre (N/m)

ip = peak short-circuit current in kiloamp (kA)

S = distance between the centrelines of the conductors in metres (m)

Once Ft in N/m has been determined then the force for each potential cable cleat can be calculated.

Metric ladder typically has rungs at 300mm intervals, so cable cleat spacing is usually a multiple of this distance. So, Ft x 0.3 gives the force a cable cleat will see if spaced at 300mm,

Ft x 0.6 for 600mm etc. Ft x cleat spacing can then be compared to the maximum recommended mechanical loop strength of the cable cleat and then the cable cleat type and spacing can be selected.

Please refer to the Ellis Patents Black Book for more examples and information on the calculation of cable cleat spacing and selection of cable cleat type.

ALWAYS REMEMBER: Whole job cost should always be considered as costs can often be reduced by using a stronger, more expensive cable cleat at a wider spacing than a cheaper option at more regular intervals.

Cable cleats are generally fastened around the cable by a threaded bolt and nut, and the higher the torque when closing this fixing, the tighter the cable cleat’s grip on the cable.

A tight grip can be advantageous when considering axial slippage, but care needs to be taken as over-tightening can lead to damage to both the outer jacket and the construction of the cable.

As a rule of thumb, cable cleat fixings should be tightened until the cable cleat is tight around the cable without any gaps between the liner of the cable cleat and the cable itself, and with no visible damage or bulging to the outer jacket.

Specific torque recommendations can be provided upon request.

1. Calculate the system peak fault current.

2. Identify the specification of the cable to be used including the nominal cable diameter and the manufacturing tolerance.

3. Identify the support structure type e.g. Ladder ( including rung spacing), basket, individual steel structure etc.

4. Consider support structure material type e.g. stainless steel, galvanised steel etc.

5. Consider the environmental conditions.

6. Consult with Ellis Patents who will provide you with the most cost effective solution for your needs.

Cable cleats and… Operating Temperatures

Our standard ranges of cable cleats are designed for use in ambient temperatures ranging from -50°C to +60°C and with cable conductor temperatures up to 90°C.

Cable cleats and… Eddy Currents

Ferro-magnetic materials that completely surround single conductors in AC circuits are susceptible to heating from eddy currents.

Generally, eddy current generation at mains frequencies requires a complete electrical and ferro-magnetic circuit around each conductor. But in installations where all three phases are contained within the same cable cleat e.g. three cables in a trefoil cable cleat, the magnetic fields of the phases cancel each other out, which in turn negates the eddy currents and the heating effect.

Despite this, it is preferable to use cable cleats manufactured from non-magnetic materials such as aluminium, injection moulded polymers or stainless steel, which has only very slight magnetic properties.

ALWAYS REMEMBER: When using single cable cleats manufactured from ferro-magnetic materials care should be taken to avoid forming a complete iron loop around the cable.

Cable cleats and… Multi-core Cables

There is a commonly held belief within the electrical industry that multi-core cables will protect themselves in the event of a short-circuit, meaning their installation does not require fault rated cable cleats.

However, research shows that the forces between the conductors of a multi-core cable in the event of a fault are similar to those between three separate single core cables laid in a trefoil arrangement.

Therefore, when specifying multi-core cables it is advisable that cable manufacturer should be contacted to ascertain the ability of its specific cable to withstand these forces.

It is worth noting that whatever the withstand quoted, in the event of a significant fault an unrestrained multi-core cable will move.

Furthermore, the requirements of most wiring regulations are clear and typically state that:

“Every conductor or cable shall have adequate strength, and be so installed as to withstand the electromagnetic forces that may be caused by any current, including fault current.”

Cable cleats and… Fire

There are currently no European or IEC standards for fire rated cable clamps, although there are requirements within other standards that can be followed to prevent unsuitable products being specified.

The international standard IEC 61914 requires non-metallic and composite cable cleats to have adequate resistance to flame propagation.

UL94, the standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances, is a plastics flammability standard that classifies plastics according to how they burn in various orientations and thicknesses. Adherence to its V-0 rating for polymers should be demanded by specifiers.

The use of the description LSF (low smoke and fume) is common terminology with regard to polymers, but is misleading as it doesn’t relate to any published standard and so can be interpreted in a wide variety of ways.

To ensure complete assurance of performance in a fire, all Ellis plastic products have undergone testing at the Building Research Establishment (BRE) in line with the London Underground 1-085 specification with regard to:

Smoke emission

Limited oxygen index

Toxicity of fumes

The appropriate products are listed in the London Underground Approved Products register. Identification numbers are 360, 361,362, 363, 364, 365 and 1661.

A great deal of focus is placed on fire rated (FP) cables and their performance in fire, but very little attention is given to the cable fixings used to secure these cables. Given that FP cable is typically rated for operation in temperatures ranging from 850°C to 950°C then the use of plastic cable cleats or clamps is clearly inappropriate.

Even aluminium only has a melting point of 660˚C, which means it would fail to support FP cables in a fire.

To counteract this shortcoming, Ellis manufactures the Phoenix range of clamps for use with FP cables. Independently tested by Exova Warrington fire and BRE, all products in the range are proven to perform to the same level as the FP cables ensuring continuous operation in the event of fire.

Cable cleats and… UV Resistance

While wholly metal cable cleats are impervious to UV attack, composite and polymer cable cleats can be at risk.

Ellis composite cable cleats such as Emperor, Vulcan and Atlas all have polymer liners, but are designed to be impervious to UV attack because the polymer is shielded by either the cable cleat’s body or the installed cables.

Polymer cable cleats that are likely to be exposed to UV should be supplied in materials containing carbon black or other UV stabilised material.

ALWAYS REMEMBER: All cable cleats supplied by Ellis for applications involving UV are provided in UV resistant materials.

Does Ellis provide any advice on cable cleat system designs?

As a cable cleat manufacturer, we do not offer advice on the design principles and choices between different types of cable installation. We will however provide expert advice on the suitability of particular cable cleats within any type of installation.

Flexible and Rigid Cable Installations

On most projects a major consideration is the constant movement of the cable due to thermo-mechanical effect. To accommodate this two principal types of installation design exist:

FLEXIBLE SYSTEMSwhere the cables are “snaked” either vertically or horizontally. The cable can expand and contract freely between fixing points.

RIGID SYSTEMSwhere the cables are rigidly fixed. The longitudinal thermo-mechanical force is withstood by the combination of the stiffness of the cable, the cable cleat reaction force and the rigidity of the support structure.

Cable cleats are designed to withstand the forces exerted by the cable in the ‘axial’ direction, this is relevant to both flexible and rigid systems. It is also important when the cables are installed vertically.

Flat, Trefoil and Quadrafoil Installations

Cable arrangements for three phase installations utilising single conductor cables are typically flat spaced, flat touching or trefoil.

The 17th Edition Wiring Regulations (BS7671) provides current ratings and voltage drop values for all these arrangements. It also contains information on grouping factors and spacing between circuits to achieve thermal independence.

Additionally, IET Guidance Note No. 6 delivers valuable advice on installation arrangements where there are multiple cables per phase.

An additional method for installing single-core cables is to use quadrafoil cable cleats where the neutral is bundled with the three phase conductors. In this arrangement, there is no advice in BS7671 but a report produced by ERA on behalf of Ellis delivered the following guidelines:

Current ratings, given in BS7671, for cables in touching trefoil formation are appropriate for cables in quad bundles

Derating factors, given in BS7671, for cables in touching trefoil formation are appropriate for cables in quad bundles

Voltage drops for circuits in quad formation should be calculated using the values tabulated in BS7671 for cables in flat touching formation

When considering multiple cables per phase, the advice given in Guidance Note No.6 for trefoil groups is applicable to quad bundles

The induced voltage in the neutral conductor of a quad group is minimal and can be ignored.

How do I prevent thread galling when installing cable cleats?

Stainless steel fasteners have a propensity to “pick-up” when the two threaded surfaces slide against each other. If sufficient speed and pressure is applied to the sliding surfaces then they can weld themselves together – a phenomenon known as thread galling.

All stainless steel fixings will thread gall if there is sufficient friction.

To avoid thread galling, reduce the speed and downward pressure when closing fasteners and use lubrication where appropriate.

“A cable cleat is a device designed to secure electrical cables when installed at intervals along the length of the cables” – IEC 61914 Cable Cleats for Electrical Installations

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“Cable cleats should:

Prevent excessive cable movement due to fault-current magnetic forces

Be rated for specific cable size and available current.”

The fifth edition of API Recommended Practice 14F (Design, Installation and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities).

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“Single core electric cables are to be firmly fixed, using supports of strength adequate to withstand forces corresponding to the values of the peak prospective short-circuit current.” – Lloyds Register. Rules and Regulations for the Classification of Ships, Part 6, Control, Electrical, Refrigeration and Fire.

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“Cables are to be installed and supported in ways to avoid chafing and undue stress in the cable.” – ABS Steel Vessel Rules 4-8-4/21.9 Cable Support, 4-8-4/21.9.1 General and 4-8-4/21.9.3 Clips, Saddles, Straps

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“In order to guard against the effects of electro dynamic forces developing on the occurrence of a short-circuit or earth fault, single core cables shall be firmly fixed, using supports of strength adequate to withstand the dynamic forces corresponding to the prospective fault current at that point of the installation.” DNV Rules for Ships / High Speed Light Craft and Naval Surface Craft, Pt. 4 Ch. 8 Sec.10 – page 68, C50

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ALWAYS REMEMBER: Any power cable system designer or installer has an obligation to consider the method of securing cables in order to restrain their movement whether caused by an electrical fault or any other reason – a job that can only be safely and securely done with correctly specified cable cleats.

IEC 61914 Cable cleats for electrical installations outlines a series of tests that can be used to assess the performance of a cable cleat’s design.

Although the standard does not define pass or fail levels, it allows manufacturers to define the performance characteristics of their products, and specifiers to compare products from different manufacturers.

The aspects of construction and performance covered by the standard include:

• Material type – i.e. metallic, non-metallic or composite

• Minimum and maximum declared service temperatures

• Resistance to impact at the minimum declared operating temperature

• The ability of the cable cleat to withstand axial slippage forces

• Resistance to electro-mechanical forces – i.e. the ability of the cable cleat to withstand the forces between the cables in the event of a short-circuit

• Resistance to UV and corrosion

• Flame propagation

The international standard IEC 61914 includes a formula in Annex B that enables a designer to calculate the force between two conductors during a fault.

If the strength of a particular cable cleat is also known, then the optimum spacing of the cable cleat along the cable in order to restrain the force created by the fault can be calculated.

The strength of a cable cleat is often determined using a mechanical tensile test. However, the results may be misleading because the force is applied in a slow and controlled manner, which does not replicate fault conditions.

In a short-circuit fault the forces are applied almost instantaneously and oscillate in every direction. Experience shows that a cable cleat that survives a mechanical tensile test at a given force will not necessarily survive a short-circuit test, even if forces are the same.

Consider the properties of glass, immensely strong under tension… but, subject to brittle failure when impacted.

When correctly specified and installed, cable cleats will restrain cables that are subjected to the forces that result from a short-circuit that is within the maximum system design fault current.

Conducting a short-circuit test is the only reliable way of proving that a cable cleat is capable of withstanding a specific set of fault conditions.

We always recommend that any claims of cable cleat strength should be supported by a short-circuit test carried out in an independent and accredited laboratory and appropriately certified.

Specifiers, consultants and engineers should also request, as standard, a complete test report that includes before and after photographs, and a table of results and conclusions.

This practice is becoming commonplace, but prior to the publication of the international standard IEC 61914 many cable cleats were not tested, and those that were had no standardised testing method by which to gauge success or failure. As a result, test results were open to a wide range of differing interpretations.

IEC 61914 has provided a standardised method for conducting a short-circuit test and a definition of the criteria for a pass. It does though allow for a significant degree of latitude and so caution must be employed when interpreting results. Note should also be taken of the full report as opposed to just its headline page.

EXAMPLE

Two manufacturers have tested cable cleats to the international standard IEC 61914 and both claim their cable cleat is capable of withstanding a peak short-circuit current of 140kA.

Manufacturer ‘A’ conducted a test using a 35mm cable cleated at 600mm centres

Manufacturer ‘B’ conducted a test using a 45mm cable cleated at 300mm centres

Your system peak fault level is 60kA, you are using a 30mm diameter cable and you wish to cleat at 1200mm centres.

Are both cleats suitable? No.

Using the formula from The international standard IEC 61914 (provided and explained in the following section) the force each cable cleat was subjected to was:

Manufacturer ‘A’ – 57kN

Manufacturer ‘B’ – 22kN

You require – 24kN (min)

Manufacturer B’s product does not meet the requirement.

ALWAYS REMEMBER: Ellis cable cleats are all short circuit tested and will meet the specific project requirements

There is a major difference between the short-circuit withstand requirements of a cable and the short-circuit withstand of a cable cleat.

The former is concerned with cable degradation as a result of temperature rise (thermal stress heating), while the latter is concerned with cable retention as a result of electromechanical forces.

Typical installation specifications that have been derived from the thermal withstand of the cable would require a short-circuit withstand of 63kA for 1 second or 40kA for 3 seconds.

A short-circuit test for a cable cleat does not consider this heating effect, and instead concentrates entirely on the destructive electro-mechanical forces at peak, followed by a short term decaying RMS.

The international standard IEC 61914 requires a short-circuit test duration of just 0.1 second. This equates to five complete cycles, by which time the true strength of a cable cleat will be known.